1. Field of the Invention
The present invention relates to a hydraulically operated braking system including a hydraulic pump operated by an electric motor.
2. Discussion of the Related Art
One example of a hydraulically operated braking system of the type indicated above is disclosed in Japanese Patent Application No. 10-243036 which was filed by the assignee of the present application and has not been laid open at the time the present invention was made. This braking system includes a wheel brake cylinder operated by a pressurized fluid to brake a vehicle wheel, a hydraulic pump for delivering the pressurized fluid to the wheel brake cylinder, an electric motor for operating the hydraulic pump, and a motor control device for controlling an electric current applied to the electric motor. In this braking system, the fluid pressure in the wheel brake cylinder is controlled by controlling the amount of the electric current applied to the electric motor. The amount of the electric current applied to the electric motor is increased to increase the output of the electric motor, when the required fluid pressure in the wheel brake cylinder or the required rate of increase of the fluid pressure is relatively high, for applying an abrupt brake to the vehicle wheel, for example.
A PWM (pulse width modulation) control is known as a common method of controlling the amount of the electric current to be applied to the electric motor. In the PWM control, a non-contact switching element included in the motor control device is alternately turned ON and OFF at a controlled duty ratio to control the amount of the electric current to be applied to the electric motor. The duty ratio is a ratio of the ON time during which the switching element is held ON, to the cycle time which is a sum of the ON and OFF times. The amount of the electric current applied to the electric motor is increased with an increase in the duty ratio. For minimizing the operating noise of the electric motor, it is generally known that the switching frequency of the switching element (namely, the PWM control frequency) is desirably higher than the upper limit of the audible range, that is, higher than the upper audibility limit. It is also known that the amount of heat generated by the switching operation of the switching element increases with an increase in the duty ratio and an increase in the switching frequency, and that the expected service life of the switching element is prolonged with a decrease in the amount of the generated heat.